There are many instances when it is desirable or necessary to know the rate of flow of blood through a particular blood vessel of a patient for diagnostic purposes and in preparation for a surgical procedure. This flow rate may vary due to various factors such as the patient's blood pressure, presence of arthroscopic plaques which may partially occlude a blood vessel, etc.
The conventional method of measuring blood flow utilizes so-called thermodilution. An intravascular catheter carrying a temperature sensor such as a thermister or thermocouple is introduced into the pulmonary artery. Then saline is injected into the right atrium or ventricle to allow proper mixing of saline in the blood. Sometimes the saline is a room temperature but more often it is at a lower temperature, e.g. 4 C to increase the temperature difference from the patient's normal core temperature. The distance from the point of saline rejection to the sensor in the catheter is known more or less. Each time a saline bolus is injected, a clock is started to measure the time it takes for that bolus to flow to the sensor which thereupon emits a signal to stop the clock. The flow rate is determined by the dividing that fixed distance by the measured time interval.
In some situations, it may not be desirable to inject saline solution into the patient's blood stream. This is particularly so for neonatates whose blood volume is small. In addition, the distance between the point of saline injection and the sensor is not always known exactly. Those prior flow rate measuring devices which utilize temperature sensors such as thermisters and the like are also disadvantaged in that the sensors are not particularly sensitive and require recalibration because their thermal resistive characteristics may change over time.